Basic Organic Concepts

Inductive Effect

It is just like shifting of shared pair of electrons in polar covalent molecules. If shared pair is more shifted towards the more electronegative atom, the less electronegative atom acquires slight positive change and more electronegative atom acquires partial negative charge, e.g.

It is a permanent effect and propagates through carbon chain. Atoms or groups having greater electron affinity than hydrogen, are said to have electron attracting or negative inductive effect (-I) while that having, smaller electron affinity than hydrogen are said to have electron releasing or positive inductive effect (+I) e.g.

Here, Cl has –I effect and alkyl group has +I effect.

Order of groups producing –I effect is

R₃N⁺ > NO₂ > CN > SO₃H > CHO > CO > COOH > F > Cl > Br > I > OH > OR > NH₂ > C₆H₅ > H

Order of groups producing +I effect is

O^- > COO^- > 3º alkyl group > 2º alkyl group > 1º alkyl group > CH₃ > H

Application of Inductive Effect

1. Presence of groups showing +I effect increases the stability of carbocation while presence of groups showing –I effect decreases their stability.
2. Strength of acid increases with the attachment of group showing –I effect and decreases with the attachment of group showing +I effect.
3. Presence of +I effect showing groups increases the basic strength of amines.
4. Reactivity of carbonyl compound is increased by –I effect showing groups.
5. Reactivity of alky halides towards S_N1 is increased by +I showing groups.

Electromeric Effect

It is defined as the polarity produced in a multiple bonded compound as a reagent approaches it. In the presence of attacking reagent, the two $\pi$ electrons are completely transferred to any of the one atom. This effect is temporary.

This may be of +E type (when displacement of electron pair is away from the atom or group) or of –E type (when the displacement is towards the atom or group). e.g.

Hyperconjugation

It involves delocalisattion of $\sigma$ electron of a C–H bond of an alkyl group attached directly to an atom of unsaturated system or to an atom with an unshared p-orbital.

This effect is also called no bond resonance or Baker Nathan effect.

Applications of Hyperconjugation

1. Stability of alkenes More the number of $\alpha$-hydrogen atoms, more stable is the alkene.

1. Stability of carbocation Greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the stability.

Resonance Effect

When all the properties of a molecule cannot be shown by a single structure and two or more structures are required to show all the properties of that molecules, then the structures are called resonating structures or canonical forms and the molecule is referred as resonance hybrid. This phenomenon is called resonance.

In resonance,

1. The arrangement of atoms must be identical in all the formulae.
2. The energy content of all the canonical forms must be nearly same.
3. Each canonical form must have the same number of unpaired electrons.

It involves delocalisation of $\pi$ electrons. This effect may be of +R type or –R type.

Positive Resonance Effect (+R)

Electron donating groups with respect to conjugate system show +R effect. Central atom of functional groups should be more electronegative than the surrounding atoms or groups to show +R effect. E.g. halogens, –OH, –OR, –OCOR, –NH₂, –NHCOR etc.

Electron donating groups producing, +R effect are ortho and para directing. They activate the benzene ring towards the electrophilic substitution reactions except halogens. Halogens slightly deactivate the benzene ring towards the electrophilic substitution reaction. More the E.D.G. more is the basic nature.

Negative Resonance Effect (-R)

Electron withdrawing groups with respect to conjugate system show –R effect. Central atom of functional groups should be less electronegative than surrounding atoms or groups to show –R effect. e.g. halogens, -COOH, -COOR, -CHO, -CN, -NO₂ etc.

Electron withdrawing group (E.W.G.) producing –R effect are meta directing. They deactivate the benzene ring towards the electrophilic substitution reaction. More the E.W.G. more is the acidic nature.

Stability of Canonical Forms

It can be judged by the following rules:

1. Non-polar structure is more stable than the polar structure.
2. Among polar structures, structure with maximum number of covalent bonds is most stable.
3. The structure with maximum charge separation is more stable.
4. Structure with positive charge on more electropositive element and negative charge on more electronegative element is more stable.

Resonance and Bond Order

Bond Order = $\frac{\text{Total no. of bonds between two atoms}}{\text{Total no. of resonating structures}}$

BO=$\frac{2+1}{2}$=1.5